Fossil fuel fired boiler systems, such as those found and used in conjunction with, for example, electric power generation systems typically expel exhaust by-products resulting from the combustion of fossil fuel. This exhaust (or flue gas) often contains various chemical compounds, such as, for example, sulfur dioxide (SO2) that are not desirable for releasing into the atmosphere for general air flow dispersal. As a result, systems are provided to remove certain chemical compounds from the flue gas stream from a fossil fuel fired boiler before the flue gas is released into the atmosphere. Such systems include, for example, wet flue gas desulphurization systems (WFGD) that are configured to remove at least a portion of sulphur dioxide (SO2) that may be contained in the flue gas stream, before it is released into the atmosphere.
In wet flue gas desulphurization (WFGD) systems such as the one depicted in the diagram of FIG. 1, an absorber tower 50 is provided and configured to subject a flue gas stream (FG) to an alkaline slurry 130 that captures at least a portion of SO2 that may be present in the flue gas stream. The absorber tower 50 has an upper section 52 and a tank section 55. The upper section 52 houses a plurality of spray heads 120 configured to spray the slurry into the flue gas stream (FG) as it elevates through the upper section 52 to exit the absorber tower 50. Slurry is pumped to the spray heads from near the bottom of the tank section 55 via one or more pumps 141, 142 and/or 143. As the flue gas FG comes in contact with the slurry, SO2 in the flue gas is captured by the alkaline slurry.
With reference to FIG. 1B and FIG. 1C, tank section 55 may be configured to include a series of air outlets 112 for introducing air into the slurry contained in the tank section 55. This air is provided to help speed up the neutralization of SO2 contained in the slurry. Each of the air outlets 112 is configured to include one or more orifices for releasing air into slurry contained within the tank section 55. The upward migration of these air bubbles occurs within the center portion of the tank section 55, away from the wall(s) 57. As a result the force of the upward migrating air bubbles tends to push fresh slurry near the top of the tank section 57 outward away from the center area of the tank section 55 and toward the wall(s) 57. The migration of air bubbles and the flow of fresh slurry is generally depicted in the diagram shown in FIG. 1D.
FIG. 1B is a diagram generally depicting the tank section 55 of the absorber tower 50. In this example, the interior wall(s) of the tank section 55 is not congested with accumulated scaling.
The example depicted in FIG. 1B and FIG. 1C shows a tank section 55 that is configured to include a pipe grid 112 for introducing a gas, such as air, into the slurry contained in the tank section 55. Each of the air outlets 112A-112E is configured to include one or more orifices 116 for releasing air into slurry contained within the tank section 55. These air outlets 112 are provided to help evenly distribute the air throughout the slurry as fine bubbles which travel to the liquid surface through buoyant forces.
With reference to FIG. 1A and FIG. 1D the general function of the WFGD will be discussed. After slurry is sprayed into the upper section 52 of the absorber 50 via the spray heads 120, the slurry comes into contact with flue gas flowing through the absorber tower 50. This slurry makes contact with the flue gas and captures at least a portion of the SO2 that may be contained in the flue gas. This slurry with newly absorbed SO2 (fresh slurry) falls downward toward and into the tank section 55. When the fresh slurry enters the tank section 52, the newly absorbed SO2 that is in the alkaline slurry is not yet completely neutralized. Until the SO2 reacts with oxygen and calcium carbonate and forms a stable gypsum crystal, the solution may become supersaturated with respect to either calcium sulfite or calcium sulfate. In this supersaturated state, the solution is prone to cause scaling to occur on interior surfaces of the tank section 55.
With reference to FIG. 1A, FIG. 1D & FIG. 1E, the fresh slurry resides in the tank section 55 until it is pumped out or re-circulated by circulation pump 141, 142 & 143 back to the spray heads 120 in the upper section 52. While in the tank section 55, the fresh slurry eventually, over time, migrates downward toward the bottom of the tank section 55. By the time fresh slurry reaches near the bottom of the tank section 55, the solution approaches equilibrium and thus the slurry is no longer as saturated as when it first entered the top of the tank section (as fresh slurry). Thus, this “aged slurry” tends to be less likely to react with the interior wall of the tank section 55 and, as a result, does general result in the build up of much, if any, scaling on the interior wall.
FIG. 1E is a diagram depicting the tank section 55 after scaling 65 has accumulated on the interior wall 57. FIG. 1E is a diagram depicting a partial cut-away view of the tank section 55 after scaling 65 has accumulated on the interior wall 57.
With reference to FIG. 1E and FIG. 1F, in the absorber tower 50, it is common for scaling to occur and accumulate along the interior wall(s) 57 of the absorber tower 50 due to the supersaturated nature of the slurry freshly subjected to the flue gas stream (fresh slurry). This scaling typically occurs in and around the tank section 55 of the absorber tower 50; however it is not limited to this area alone.
From time-to-time, accumulated scaling must be removed from the absorber tank wall(s). In order to remove the scaling, it is typically necessary to shut down the WFGD system 25 and manually enter the absorber tank 50 to physically remove the scaling from the walls(s). This often requires the set up and subsequent removal of equipment, such as, for example, scaffolding and safety equipment within the interior of the absorber tank to allow personnel to reach areas on the absorber tank interior wall where scaling has occurred and safely remove it.
Shutting down the WFGD, as well as setting up systems for personnel to use in removing the scaling from the interior walls is a time consuming and expensive endeavor. Further, introducing personnel into the enclosed space of the absorber tank subjects them to potential safety risks associated with scaling falling from the interior walls.